The present invention relates to a photopolymerizable composition which can be applied, particularly in the form of a dry-resist layer, to the surface of a printed circuit, especially to a solder-resist layer, and which is capable of being processed into visible markings.
The surface of printed circuits, to which compounds are to be applied, is often provided with markings or characters that assist in the application procedure. Markings are usually applied to a solder resist which has been coated upon the printed circuit board, leaving the solder pads uncovered. Basically, pigmented marking resists, which are applied according to the screen-printing process, are used for this purpose.
The marking resist must have a high adhesive strength with respect to the base, in particular to the solder resist. The best results are obtained with two-component resists. However, the usable life or pot life of a two-component resist after mixing generally is no longer than 6 to 8 hours, i.e., the resist can only be used within this period of time. After this time, the mixture will gelatinize and become solid. Marking resists have been used that are based on printing inks which are curable by ultraviolet light, but to date the desired adhesive strength has not been attained with resists of this kind. When these resists are printed on a completely polymerized solder resist having a high surface gloss, the possibility of anchoring is reduced.
In addition, the resistance of markings cured by ultraviolet light is often insufficient under the conditions of subsequent processing, which can include, for example, a soldering step and a subsequent cleaning, under the action of ultrasonic waves, using aggressive chemicals and cleansing agents. Settling of the pigment in storage is also a disadvantage. Moreover, in many cases where only small numbers of workpieces have to be marked, it is too expensive to prepare a screen-printing stencil solely for marking.
On the other hand, it is known, for example, from U.S. Pat. No. 4,438,189, to produce solder masks according to the dry-resist process. In this process, a photopolymerizable layer comprising polymeric binders, polymerizable compounds, photoinitiators, and thermally activatable crosslinking agents is laminated onto a printed circuit, imagewise exposed, such that the solder pads are left unexposed, developed and then thermally cured. These layers usually contain a dye which renders the developed solder mask visible. It is also possible to add dyes which are bleached or which change color on exposure, in order to be able to differentiate between exposed and unexposed area. Care has to be taken, however, to prevent any adverse influence on the photosensitivity of the photopolymerizable layer. Therefore, dyes are used that absorb the smallest possible amount of light in the short-wave spectral region, and these dyes are added only in relatively low concentrations.